As larger-sized liquid crystal display panels have come to be widely used in recent years, the one-drop filling (ODF) method for forming a liquid crystal layer between a pair of substrates is replacing the conventional vacuum injection method. The one-drop filling method includes the following steps:
(1) On one of the pair substrates, a pattern is formed using a sealant, surrounding the region in which the liquid crystal layer is to be formed. The sealant pattern is formed by drawing a pattern using the sealant supplied by, for example, a dispenser.
(2) A liquid crystal material is dropped in the region surrounded by the sealant pattern.
(3) Under a low pressure atmosphere, the other substrate is bonded on the aforementioned substrate with the liquid crystal material interposed therebetween. Then the sealant is cured.
Since the pattern formed of the sealant must completely surround the region in which the liquid crystal layer is to be formed with the one-drop filling method, at least one joint must be formed in the sealant pattern.
Characteristics of the sealant pattern formed with the one-drop filling method will be described with reference to FIG. 3.
FIG. 3 shows an example of four pieces of liquid crystal display panels manufactured using a mother substrate. The mother substrate 20 includes four opposite substrates (typically, color filter substrates), and each display region 20D includes an opposite electrode (not shown) and color filters (not shown) arranged with the corresponding pixels. The same reference numeral is used for the mother substrate and the opposite substrate. Furthermore, the opposite substrate 20 includes a black matrix (light shielding layer) 22, which surrounds the display region 20D, and the black matrix 22 defines the outer edges of the display region 20D. Although FIG. 3 illustrates TFT substrates 10 as separated for each liquid crystal display panel, actually, a mother substrate having four TFT substrates 10 thereon, similar to the case with the mother substrate 20, is prepared and coupled to the mother substrate 20. Necessary circuit elements, including TFTs, pixel electrodes, gate signal wiring, and source signal wiring, for example, are formed on each display region 10D of the four TFT substrates 10, respectively. The TFT substrate 10 and the opposite substrate 20 are fixed to each other by a sealant portion 32. The sealant portion 32 is formed outside the black matrix 22. Glass substrates for the TFT substrate 10 and the opposite substrate 20, are referred to as “glass substrate 11” and “glass substrate 21,” respectively.
A portion outside the display region 10D or 20D on the liquid crystal display panel is called a non-display region (or a frame region) and is preferably made as narrow as possible. The black matrix 22 and the sealant portion 32 are disposed in the non-display region.
On the other hand, the black matrix 22 must have enough width in order to prevent any extraneous light from entering the display regions 10D and 20D. Inadequate light shielding would lower the quality of the black color display and have a significant effect on the image quality. In order to satisfy both of these requirements, the sealant portion 32 must be formed very close to the outer edges of the black matrix 22 with a high degree of precision.
The sealant portion 32 formed by the drawing method, however, includes at least one joint portion 32b. This joint portion 32b of the sealant tends to become thicker than a sealant extension portion 32a. Here, the sealant extension portion 32a refers to the sealant portion excluding the joint portion 32b, and the width of the sealant extension portion 32a is fairly constant. The extension portion 32a is where a sealant pattern is drawn with a sealant using a nozzle of a dispenser, for example, as it moves relative to the substrate surface. Therefore, the resulting width of the sealant extension portion 32a is determined by the rate of deposition of the sealant and the speed of the nozzle movement. For this reason, the width of the extension portion 32a is stable. On the other hand, the joint portion 32b includes a portion where the sealant is first deposited (sealant pattern start point). The amount of sealant initially deposited on the start point depends on the amount of the sealant that has accumulated at the tip of the nozzle. This, in turn, is affected by a variation in the time required for positioning the nozzle (including height positioning) and a variation in the amount of sealant left in the nozzle tip when the nozzle leaves the substrate at the end point of the sealant pattern. Since the amount of sealant deposited at the start/end points of a sealant pattern drawing is variable as described above, and a joint must be formed there, the width of the joint portion 32b tends to become wider than that of the extension portion 32a. 
FIG. 4(a) and FIG. 4(b) are enlarged views of the sealant joint portion and its vicinity. FIG. 4(a) is a plan view, and FIG. 4(b) is a cross-sectional view taken along the line 4B-4B′ in FIG. 4(a).
As described above, when the sealant pattern becomes wide at the joint portion 32b, that portion of the sealant pattern can overlap the black matrix 22. Sealants containing photocurable resin (including those that are also thermally curable) are widely used, and when such sealant is exposed to radiation (typically ultraviolet (UV) radiation) from the side of the opposite substrate 20, a portion 32′ of the sealant that overlaps the black matrix 22 is not adequately cured, and as a result, the uncured components of the photocurable resin can dissolve into the liquid crystal material. Such dissolved components, in particular ionic components, can cause a reduced reliability of the liquid crystal display panel, such as decreased ability to hold voltages and alignment failure.
Furthermore, the sealant pattern width tends to become wider at corners of a sealant pattern, because the movement of a nozzle of a dispenser, for example, slows down there. Therefore, the problem that a portion 32′ of the sealant that overlaps the black matrix 22 as shown in FIG. 5(a) and FIG. 5(b) cannot be adequately cured sometimes occurs at the corner portions of the sealant patterns as well. Here, FIG. 5(a) and FIG. 5(b) are enlarged views of a corner portion of a sealant pattern and its vicinity. FIG. 5(a) is a plan view, and FIG. 5(b) is a cross-sectional view taken along the line 5B-5B′ of FIG. 5(a).
The inventor of the present invention disclosed in Patent Document 2 a liquid crystal display device in which the width of the sealant pattern is prevented from becoming wider by providing a region(s) having an extra space between a CF substrate and a TFT substrate (hereinafter “wide-gap region”) at the joint portion of the sealant pattern or at the corners of the sealant pattern. The wide-gap regions are formed by providing a recess(es) in the surface of the opposite substrate or the TFT substrate, on the side facing the liquid crystal layer. In the wide-gap region(s), a portion of the sealant is absorbed inside the recess(es), which prevents the sealant from spreading inward on the substrate surface. An example of the method for forming a recess on the surface of the substrate is disclosed, in which a recess(es) or a through hole(s) is(are) made at a predetermined location(s) on a resin film formed on the TFT substrate. The entire contents of Patent Document 2 are hereby incorporated by reference.